The present invention relates generally to electrical power supplies. More particularly, it relates to a method and apparatus for reducing dead time and reverse recovery losses in buck regulators to provide increased efficiency and higher operating frequency.
Buck regulators are commonly used in electronics for changing the voltage or polarity of a power supply. Buck regulators typically employ two electronic switches (typically MOSFETs) in combination with an output inductor. The switches are alternately turned on, thereby providing voltage pulses to the output inductor.
During portions (dead times) of the switching cycle, both switches are off. When both switches are off, the output inductor produces freewheeling current that flows through the integral body diode of one of the switches. Body diode current produces substantial energy loss due to the forward-bias voltage drop across the diode, thereby reducing the energy efficiency of the buck regulator. Additionally, reverse recovery loss occurs when the body diode later becomes reverse-biased. In order to improve the efficiency of buck regulators, body diode conduction and reverse recovery losses must be reduced.
In order to provide buck regulators with small size and weight, and reduced cost, it is best to operate the buck regulator at high frequency. However, energy loss from both body diode conduction and reverse recovery increase dramatically with increasing operating frequency. In this way, body diode conduction and reverse recovery tend to limit the maximum operating frequency of buck regulators.
It would be an advance in the art of buck regulator design to provide a buck regulator having greatly reduced dead time, and a greatly reduced or eliminated reverse recovery loss. Such a buck regulator could operate with very high efficiency compared to conventional buck regulators, and could operate at high frequency. High frequency capability can provide many benefits such as smaller size and weight, and reduced cost.
The present invention includes a buck regulator having an output inductor, first and second switches, a voltage sensor, and a gate controller. The first and second switches and the inductor are connected at a swing node. The voltage sensor is connected to the swing node and senses a voltage at the swing node. An output of the voltage sensor is connected to the gate controller. The gate controller controls the second switch in response to signals from the voltage sensor.
The gate controller may also control the first switch. In one embodiment, the voltage sensor triggers the gate controller in response to a voltage reversal of greater than 0.3 volts at the swing node.
The voltage sensor can comprise a bipolar transistor with an emitter connected to the swing node, and a collector connected to the gate controller. A base of the bipolar transistor may be connected to a voltage within about 0.7 volts of ground potential. The bipolar transistor may be made from a parasitic bipolar transistor integral to a lateral diffusion (LD) MOSFET, commonly used in power integrated circuits.
The voltage sensor can also comprise a first sense MOSFET and a second sense MOSFET connected in series. The first sense MOSFET is connected to the swing node, and the second sense MOSFET is connected to the gate controller. Also, a current amplifier can be connected between the second sense MOSFET and the gate controller.
The gate controller may control both first and second switches in response to the output of the voltage sensor. Specifically, the gate controller can turn on the second switch when the voltage sensor detects a voltage reversal at the swing node resulting from turn off of the first switch. Also, the gate controller can turn on the first switch when the voltage sensor detects a voltage change close to 0.7 volts at the swing node resulting from turn off of the second switch.
In another aspect of the present invention, the buck regulator comprises an output inductor, first and second switches, a gate controller, and a voltage sensor comprising a bipolar transistor with an emitter connected to the swing node, and a collector connected to the gate controller. The bipolar transistor can comprise a parasitic bipolar transistor integral to an LDMOSFET. The base of the bipolar transistor can be connected to a voltage within about 0.7 volts of ground potential.
In yet another aspect of the present invention, the buck regulator comprises an output inductor, first and second switches, a gate controller, and a voltage sensor comprising a first sense MOSFET connected to the swing node, and a second sense MOSFET connected in series with the first sense MOSFET. The first sense MOSFET has a gate connected to a reference voltage; the second sense MOSFET has a gate connected to a ground potential, or to a voltage within 0.7 volts of ground potential. A current amplifier can be connected between the second sense MOSFET and the gate controller. The second sense MOSFET can be a MOSFET.